Wave guide construction



July 28, 1959 F. E. ASHBAUGH ETAL 2,897,461

WAVE cum: CONSTRUCTION Filed Sept. 14. 1955 A If.

INVENTOR FEED E. AS5454 PAUL E. KEN/v A TTOE/VL: V5

WAVE GUIDE CONSTRUCTION Fred Edmund Ashbangh and Paul B. Kennedy, Seattle, Wash., assignors to Boeing Airplane Company, Seattle, Wash, a corporation of Delaware Application September 14, 1953, Serial No. 379,880

4 Claims. (Cl. 33395) This invention relates to improvements in the construction of electromagnetic wave guides generally of the tubular conductor type used extensively in microwave apparatus. The invention is herein illustratively described by reference to its presently preferred form as applied to the construction of a rectangular wave guide with an integral end flange; however it will be readily appreciated by those skilled in the art that certain modifications and changes therein may be made without departing from the essential and characterizing features involved and that the novel teachings of the invention may be applied to wave guides of various configurations as well as to associated microwave components in or through which electromagnetic Wave energy is propagated or sustained. The term wave guide as used hereinafter is intended to embrace all such wave guide and associated microwave components to which the novel aspects of the invention may be applied.

An object of the invention is a wave guide of lightweight construction.

Another object is a wave guide construction simplifying the manufacture of complexly shaped wave guides and Wave guide components.

Still another object is such a wave guide which is sealed against penetration of moisture into the wave guide and associated apparatus connected thereto.

A further object is such a wave guide which will permanently retain a given effective internal pressurization insuring constancy of operation in widely varied ambient pressure, thereby eliminating the need for pressurizing equipment commonly used with conventional microwave apparatus, especially in aircraft applications.

A general object is an improved wave guide construction particularly suited for aircraft and similar microwave applications.

Conventional wave guides comprise hollow metal tubes and cavities deriving their structural strength and rigidity from a substantial metal wall thickness. Diflicult molding procedures are involved in producingaccurate internal dimensioning and these procedures became very complex when special forms are required such as bends and fittings of various types. These procedures to be economical usually involved the preliminary step of producing a disposable mold core of the desired wave guide internal form and ultimately dissolving or otherwise removing the core material from the hollow metal wave guide molded around it. Often, however, complex bends cannot be made in a unitary form but must be assembled from sections of wave guide secured together by flanged couplings.

The present invention achieving the above objects provides a wave guide construction wherein the preshaped core upon which a conductive shell is formed advantageously becomes a permanent part of the completed wave guide. In accordance with the improved Wave guide construction the desired Wave guide internal configuration is molded, cut or assembled from a solidified atent 1 2,897,461 Patented July 28, 1959 ice low density foam dielectric substance having a relatively low dielectric loss tangent, high strength and low specific weight. The form, strength and rigidity of the completed wave guide are imparted chiefly by this permanent foam plastic core. The wave guide conductor is then formed by applying a thin continuous layer of conductive metal to all longitudinal surfaces of the core. This layer need be sufiiciently thick only for electrical purposes as it is not required to add appreciable strength nor rigidity to the completed Wave guide. At very high frequencies the skin effect of electrical currents required to flow in the wave guide Wall for unattenuated propagation of energy in the guide permits the metal layer thickness to be less than a few hundredths of an inch. In the preferred form of construction this conductive layer or shell is protectively encased in a lightweight overlayer which is preferably formed by a resin impregnated Fiberglas laminate.

These and other features, objects and advantages of the invention will become more fully evident from the following description by reference to the accompany ing drawings.

Figure 1 is a perspective view of a section of rectangular wave guide with a coupling flange thereon and with portions broken away sectionally to illustrate internal construction.

Figures 2 to 5, inclusive, are longitudinal sectional views showing successive stages in the manufacture of the wave guide construction appearing in Figure 1 according to the presently preferred technique.

With reference to Figure l the completed wave guide structure illustrated by way of example comprises a solidified foam dielectric core 10 of desired rectangular cross-sectional dimensions and of any desired length and longitudinal configuration. Upon the longitudinal walls of this rigid core the thin conductive metal layer 12 is applied, and forms a continuous shell constituting a wave guide conductor for propagation of microwave energy through the length of its interior.

A metal layer flange conductor 14 is formed integrally with the end of the tubular metal shell 12. Normally the plane of this end flange conductor will be perpendicular to the longitudinal axis of the shell immediately adjoining it. The end flange conductor 14 is maintained in this relationship with the conductively coated core by backing elements preferably formed of a suitable solid plastic substance and together constituting a flange collar 16 surrounding and adhesively bonded to the end of the metal layer 12. This flange collar has bolt holes in each of its four corners by which the flanged end of the completed wave guide may be secured in conductive relationship to the flanged end of another similar wave guide or to a different microwave component such as a cavity resonator, high-frequency oscillator tube or the like. The flanged collar 16 may be moulded in one piece or as illustrated, in four separate pieces 16b, 16c, 16d and 16s adhesively bonded together at interfaces 16' coincident with opposite longitudinal walls of the wave guide.

A protective over-layer 18 surrounding the thin conductive shell is applied in the example by wrapping the same with overlapping turns of resin impregnated Fiberglas fabric tape. The Wrapping is caused to overlap the base of the collar 16 which is tapered in thickness to receive the wrapping smothly. After the wrapping is complete the resin is cured resulting in a tough and durable shield for the relatively soft conductive metal layer 12. Depending upon its thickness and make-up this laminated shield lends strength and rigidity to the completed Wave guide, although as mentioned above the permanent foam dielectric core 10 constitutes the principal structural member of the wave guide.

The various different types of low density dielectric substances which lend themselves to manufacture of the permanent core in this construction are virtually too numer u t men on E a p es o diffe en types dielec c whic ma lie-u e inc u e alky T q's' s a e polyesters, ,polystyrenes, cellulese acetateapolyvinyl chloride, styrene-butadiene copolymers, phenolic resins, urea resins,.melamine, silicones, certain ceramics and others. Of course some of theseare better than others for the purposes at hand since they vary in their capability of being molded or cut into desired shapes when foamed into a lightweight cellular state having ,a low dielectric loss factor, a 'high degree of toughness, strength ,and rigidity, non-absorbency to moistureandthe capacity to retain pocketed gas or air at constant internal pressures despite ambient pressure ehanges to which the completed wave guide is subjected. It'will be appreciated, there fore, that any dielectric substance meeting these requirements for a practicable wave guide core may be employed. The term foam dielectric is intended, to embrace all suitable core substances meeting the requirements as above indicated.

Preferably the foamed dielectric core '10 is molded, since its exterior surface is ,then smooth and the conductive metal layer 10 subsequently deposited thereon will also be smooth, hence most efiicient for its purpose as a conductor of microwave currents.

The preferred method of molding a foamed core piece having an exceptionally smooth exterior is to transfer the mixture of dielectric powder and foaming agent us'ed to a moldwhichhas been coated with a special parting film. This film is produced 'by brushing or spraying a suitable silicone resin on the mold surfaces ahd baking the same at the necessary temperature for producing a cure and-for-bonding'the cured resin film to'the mold surfaces. Qne or more coats of lacquerpr resin arefihen sprayed or brushed on this cured basic film before the foam mixture is injected into the mold. It, is 'foundthat the solidifiedfoam cured in such a mold adheres to the lacquer or resin coatings applied 'to the basic film, but .thatthese coatings do not adhere to the basic film. In this manner the" molded core is easily broken from the mold and possesses very smooth surfaces as desired. "Moreover the basic silicone resin film is unimpaired and may be used over and over again; 7

However, instead 'of one-piece molding-the core -may be machine-cutif desired or assembled from moldedor cut pieces bonded together to-build up the de sired core configuration. This latterexpedient is particularly useful informing-complex-wave guide components and-fittings and enables eliminating many of-the couplingflanges now being used in 'hollow metal wave guides many sections of which cannot readily -be molded in unitary form. Furthermore -the improved construction involving a permanent foam dielectriccoreperrnits f xing the position of a core insert component-by'its embedment in-the co re during molding or assembling operations. Metal elements producing reactance effects and-tl 1e like are often incorporated inside wave guides and the improved construction greatly simplifies their installation in most instances.

The density of thedielectric core-substance will depend of course upon the degree to which it is expanded by the particular foaming technique employed andupon the density of-the unfoamed dielectric substance. It is found that good results are achieved with any ofthe .above enumerated dielectrics if their density in solidified foamed condition approximates ten pounds per cubic foot. By good results vis meant a',favorable balance between-the factors of strength and rigidity (fostered by higher densities) on; the :onehand, and alight-weight, lowdielectric loss core, on the otherhand. The attributes ofmoisture resistance and pressurization are not affected by density of the completed core since they derivefrom the self-sealing properties of the core material filling out the entire interior of the microwave conductor shell 12 wherein intense electrical fields may exist tending to produce arcing and electrical losses in unpressurized, unsealed wave guides of conventional form.

In practical wave guide assemblies produced according to the above, weight savings ranging in the order of to percent and more are achieved on the basis of the weightof ceatentiqna h ow b a tube w guide construction. The importance of this in aircraft and sim la m stawax a as am is msr ii a A -.thin.cond c iv me a co t n 2 m b s c e fully applied to the core :10 .by any of various different techniques. Those ,presently favored are electroplating, vacuum metalizing, and spraying, the latter being preferred. A third method is to wrap the core in metal foil and spray the wrapping joints or edges with the same or a similar metal for electrical continuity. Copper, brass, aluminum, zinc or other conductive metals or alloys may be used for the conductive layer.

Figures 2 to 5 inclusive illustrate the preferred method of forming the end flange. As in Figure 2, a mandrel plate A is temporarily bonded to the end face of the core 10 and is of a size and shape corresponding to the desired flange dimensions. As in Figure 3, the metal coating 12, 14 is applied in a single operation to all sides of the core and to the back side of the mandrel plate A projecting therebeyond. The .flange collar elements 1612, 1-60, 16d and 16e are then assembled around the coated core adjacent the coated mandrel plate (Figure 4) with adhesive substance applied to all surfaces of contact so as to bond these elements to each other and to .the shell conductor '12 and flange conductor 14. The wrapping 18 is then applied and the wrapping impregnant cured. Thereupon-themandrel plate A is removed, as shown in :Figure 5, leaving the metal flange conductor 14 exposed for purposes of electrical contact in a wave guide joint.

The foregoing .detailed explanation of the preferred mode of practicing the invention is intended to convey anunderstandingof the essentials involved therein but :the .details of'illustration are not .to be construed as limiting the scope thereof.

" W'e-claimlas our invention:

.11. A waveguide for eleotromagneticwave propagation comprising an elongated continuous low-density foam dielectric core of substantially rigid predetermined wave guide form and a relatively thin conductive metallayer adhered .to the sides ofsaid core and forming a continuous shell surrounding the same insufficiently thiclctoimpart appreciable stiffness and rigidity of itself to the .wave guide, said core imparting stiffness and rigidity to .the completedwaveguide,.said wave guide additionally comprising an. end connecting flange formed-by aconductive metal flange electrically contiguous to .anend'of the con- .ductivemetalshell, and .a backing element adhered to .the. metal flange and to .the shell at the baseof said flange.

2. Thewave. guide defined-in claiml, and aprotective overlayer .surrounding and adhered to the conductive .metal s]1ell comprising a.resin-impregnated .fabric wrapplng.

3. Thamethodof manufacturing flanged wave guides ;f01', electr0magnetic.waves comprisingthe initial step of formingawave guide. core oflow-density foamed. dielec- ;tric substance having anend .faceto be connected to other electromagnetic wave guide .means, temporarily adherin tmanslr p e to ,s d, e d f ce t p j c beyon a ed e :t erec pp in .a co u ti e me a coating to all sides of said core and to the exposed portions ofithe side ofsaid plate adhered to gore to form .a ,flanged, continuous conductive. shell surrounding s idv orc afdhering a backing collar tothe flangeofsaid gr ndactive shell and ato the shell itself .surroundingsaid core at the base of such flange,, and.thereafter remeving sai audienc t ex s s he e en a fe g Jflange.

References Cited in the file of this patent UNITED STATES PATENTS Llewellyn Jan. 3, 1939 Jackson Jan. 5, 1943 Converse Oct. 10, 1944 Fox Nov. 26, 1946 Cork Feb. 24, 1948 Pfleumer Apr. 5, 1949 Rock May 13, 1952 6 Ei-tel Feb. 17, 1953 Carr Dec. 14, 1954 Rosencrans Mar. 22, 1955 Van Atta Aug. 28, 1956 FOREIGN PATENTS Great Britain Jan. 8, 1947 France Nov. 1, 1950 OTHER REFERENCES pages 13438. 

